'project/project' is valid directory for 'project' project-related files. For example, if one configure his sbt build to use scala files directly from 'project/', dependencies like 'project/project/plugins.sbt' will be ignored because of this line.
See http://www.scala-sbt.org/0.13/tutorial/Organizing-Build.html#sbt+is+recursive.
The *.iml file is IDE specific. It is generated by Intellij IDEA to store project related metadata. It is not a Play Framework specific artifact and should not be included in the PlayFramework.gitignore.
See: https://github.com/github/gitignore/pull/1581
I would also vote for the removal of .eclipse and *.eml files on similar grounds but will do so in a separate pull request.
The original ignore was far too general and results in eliminating packages
named "db" or "project", for example. These are not that uncommon for package
names. Fixes #921.